Printing units in offset printing presses usually comprise four parallel cylinders, which are mutually in surface contact through one of their generatrices. A web of printed paper runs between the two central cylinders known as blanket cylinders which transfer printing ink from plate cylinders to the running web of paper. The blanket cylinders are fitted with printing blankets formed of a cloth coated with an elastomer material. Due to the viscous characteristic of the printing ink transferred to the printed web of paper, the printed web has a tendency to adhere to the printing blankets on the blanket cylinders which are damp.
During operation of the printing press, the running web of paper is under a high tension stress created by entry rollers situated before the printing units and cooling rollers situated after the printing units and dryer. The tension in the web causes the web to peel off of the damp printing blankets. This peeling off of the web from the printing blankets is sometimes abrupt, causing the web to tear.
Furthermore, when offset printing inks of the hot-drying type are used, a hot air dryer is provided in the printing press. The hot air dryer is usually between 8 and 12 meters long and is designed to cause the solvents of the ink deposited on the web of paper to evaporate and to cause its resin to be cured. The hot air dryer is placed after the last printing unit. The printed web of paper runs through the hot air dryer without any support by rollers. Since the dried web of paper is less resilient than the damp web of paper, tearing of the web of paper may also occur in the dryer.
When the web of paper tears in the dryer, it usually winds itself onto one of the blanket cylinders in the last printing unit as a result of the adhesion of the ink deposited on the web, which can create considerable bonding forces. After a certain number of revolutions, when the number of layers of paper reaches a critical mass, the torn web of paper winding itself onto the blanket cylinder can ruin the printing blanket or even the cylinders. Before the printing press comes to a complete stop, the risk of incident is increased, since for each revolution of the blanket cylinder, two layers of paper are wound around the blanket cylinder, one of which comes from the printing unit and the other from the dryer.
A non-return device disclosed in European Patent EP-0,092,659 attempts to overcome the above-mentioned disadvantages. This device is placed between the last printing unit and the dryer. The main component of this device is a pair of gripping rollers, comprising one fixed roller laterally set underneath the running web of paper and one pressure roller disposed above the web, both rollers being driven in rotation at the running speed of the web. During normal operation when the web is running, the two rollers are separated so that the damp printed web can pass without touching them and so that there is no mackling. When the web of paper tears, the breakage of the web is detected by optical detectors placed at the output of the last printing unit and a trigger signal is instantaneously sent to a mechanism holding the pressure roller apart. The pressure roller is then released and brought into a position for gripping the torn web of paper. As long as the printing press is still running, the web of paper from the last printing unit continues to run and winds itself onto one of the two gripping rollers, instead of winding itself onto one of the blanket cylinders in the last printing unit. The diameter of the roller around which the paper winds itself increases while the other roller moves apart so as to permit the reception of the paper.
As the diameter of the roller onto which the web of paper winds itself increases, this roller has a tendency to wind an increasing length of paper web at each revolution. The roller thus exerts an increasingly greater tension on the web of paper which in itself creates the possibility of tearing. In order to avoid this, it would be necessary to fit the device with a friction clutch system. The friction clutch system would make it possible to adjust the drive of the two gripping rollers, so that the rollers would begin to slip once the tension in the torn web of paper became too great; such slippage would progressively increase as the diameter of the winding roller increased. The fitting of such a clutch system is complex and there is no guarantee that it would solve the tearing problem.
Moreover, the torn web of paper winding itself around the gripping roller is coated with damp ink and when the printing machine is stopped, it is then difficult to remove all of this paper stuck onto the roller.
Lastly, the two gripping rollers turning close to each other on either side of the running web of paper, risk coming into close contact at any moment, which presents a danger to the personnel on duty.